The present invention relates generally to class AB output stages of the type referred to as “Monticelli class AB output stages”. More particularly, the present invention relates to an improved Monticelli class AB output stage which can operate reliably with a supply voltage of approximately 1.8 volts.
The closest prior art is believed to be disclosed in U.S. Pat. No.4,570,128 issued Feb. 11, 1986 to Montecelli. “Prior Art” FIG. 1 herein shows the closest prior art disclosed in the Montecell patent. A Monticelli class AB output stage provides the fastest known “rail-to-rail” operation, but requires a minimum power supply voltage equal to the sum of 2 VGS (gate-to-source) voltages plus a VDSsat voltage, which is approximately 2.2 volts. Therefore, a typical Monticelli class AB output stage cannot be used in integrated circuits that require a lower power supply voltage of approximately 1.8 volts. Prior attempts to provide a class AB output stage which operates at lower power supplies have included negative feedback circuitry that unacceptably increases the overall loop delay.
Thus, there is an unmet need for an improved class AB output stage that reliably operates at high speed from a power supply voltage of approximately 1.8 volts.
There also is an unmet need for an output stage that is capable of sourcing and/or sinking substantial current and that is stable with any capacitive load while operating from a power supply voltage of approximately 1.8 volts.